This invention was made for the purpose of constructing a flexible tube element for the exhaust systems of combustion engines in vehicles with a helically or annularly corrugated bellows made of metal, with a stripwound metal hose in a coaxial position to the bellows and with coaxial, mostly cylindrical connection fittings installed at the ends of the bellows and/or the hose.
Such flexible tube elements are, as a rule, installed as an intermediate part in an automotive exhaust system which consists of rigid elements, for the purpose of absorbing such movements and vibrations as are caused by the elastically supported engine, by shock, by changes of length due to thermal effects, etc., and of insulating adjacent components from such movements and vibrations. The tube elements therefore not only have to have sufficient heat-insulating and sound absorbing properties and have to be tight to exhaust gases, they especially have to have suitable vibration damping properties.
Whereas the bellows, together with the connection fittings which are at least indirectly fixed to the bellows usually by welding, provides gas-tightness of the metal hose element, the stripwound metal hose is the part on which the damping properties are based. The damping of movements by this helically stripwound metal hose made of an, in general, pre-profilated metal strip by interlocking adjacent strip edges and by forming several layers with interlocked profile, is achieved by the conversion of the movements into friction, the so-called lost work of deformation, between the adjacent strip edges or layers with interlocked profile, for an angular, axial, lateral or torsional deflection. The friction values depend on the rate of winding or interlocking.
For an optimum damping of movements, the winding rate must be defined thus that there is neither almost no friction between the said hose edges due to a too loose winding which will reduce the obtainable damping properties to a very low value, nor that the winding is so tight that the friction in the metal hose is too high due to its rigidity, with the damping properties of the metal hose being reduced to a rate almost equal to the very low damping properties of a rigid pipe.
Even if a stripwound metal hose of optimum design is installed in the flexible tube element, there are still applications in which the damping properties of the metal hose cannot take full effect or in which its damping properties are yet not sufficient. The damping properties can be diminished at certain operating temperatures of the stripwound metal hose, i.e. at high temperatures of the hose due to a longer operating period of the exhaust system. One reason for this effect is the so-called thermal stiffening occurring especially in the bending zones of the stripwound hose due to the different material tensions induced during the winding process trying to relax and expanding at different rates in this process. As a result, there is an increase in friction between the adjacent strip edges or layers with interlocked profile, which will cause a restriction of the mobility of the hose or an increase in wear of the hose areas which are in contact with each other and, finally, a deterioration of the damping properties.